Since the invention of the transistor, manufacturers, researchers and users have constantly sought to improve the device by reducing its size and increasing its speed of operation. In a vertical transistor charge carriers pass from one vertical layer to an adjacent layer. The speed of such a vertical device depends upon the time that it takes for a charge carrier, such as an electron, to pass from one layer of the device to an adjacent layer.
Some semi-conductor compounds, especially those comprising group III-V elements, and in particular, compounds of gallium arsenide are capable of transferring charge carriers, such as electrons, with minimum scattering of electrons. Such type of transport of charges is commonly called "ballistic." See, Trudy E. Bell, "The Quest For Ballistic Action," IEEE Spectrum, February 1986, p. 36.
Transistors in general are broadly classified into vertical or horizontal structures. In a horizontal structure charge carriers are generated at a source and pass laterally through a channel toward a drain horizontally displaced from the source. Above the channel is a gate. A voltage on the gate controls the horizontal passage of charge carriers from the source to the drain.
A vertical transistor fabricated from gallium arsenide and aluminum gallium arsenide that exhibits ballistic electron transfer was originally proposed by one of the inventors of this invention. See, Mordehai Heiblum, "Tunneling Hot Electron Transfer Amplifiers," 24 Solid State Electronics, page 343 (1981). Since that proposal, a vertical type tunneling hot electron transfer amplifier (THETA) has been constructed and ballistic transport of electrons in such a device has been directly observed. See, Heiblum, et al. "Tunneling Hot Electron Transfer Amplifier: A Hot-Electron GA As Device With Current Gain", Applied Physics Letters, 47 November, 1985, pages 1105-1107; Heiblum, et al., "Direct Observation of Ballistic Transport in Ga As", Physical Review of Letters, 55 No. 20, November, 1985, pages 2200-2203.
The practical fabrication of THETA devices is difficult. The base electrode is a layer of very thin material, disposed beneath the emitter. Ohmic contacts, when applied to the base, diffuse into the base. The base material is normally constructed from a heavily doped gallium arsenide layer. The base regions exposed to air will have a depletion layer. Since the base is so thin, the depletion layer will greatly increase the base resistance. This will increase coupling between output and input circuits of the device and impede its frequency performance. Ohmic contacts to GaAs are usually formed by diffusing a metal into the GaAs, thus resulting in a substantial penetration depth into the base. This may result in shorting the base to the collector thereby rendering the device inoperable. Attempts have been made to overcome such problems by using relatively shallow contact techniques. Such shallow contacts do not consistently yield the desired conductor performance required and they are difficult to fabricate.